1. Field of the Invention
The present invention relates primarily to 4-stroke cycle diesel engines, particularly, engines equipped with a DPF (diesel particulate filter), variable geometry turbocharger, and internal EGR (Exhaust Gas Recirculation) system which is composed such that a fraction of the exhaust gas joins together with the charged air in the cylinder of the engine by allowing the exhaust valve to lift a little in the suction stroke.
2. Description of the Related Art
There are 4-stroke cycle diesel engines and 4-stroke cycle gasoline engines that adopt an internal EGR system in which a fraction of the exhaust gas joins together with the charged air in the cylinders of the engine by allowing the exhaust valve to lift by a small lift in the suction stroke. This is called exhaust valve sub-lift type internal EGR system.
The exhaust valve sub-lift type internal EGR system will be explained with reference to FIG. 6. In FIG. 6, valve lift curve of the exhaust valve 7 and that of the inlet valve 5 versus crank angle are indicated by Es and In respectively. The exhaust valve lift Ex in the exhaust stroke is called here as the main exhaust valve lift. The exhaust valve 7 is lifted again by a small lift Hs in the suction stroke as shown by sub-lift Es of the exhaust valve 7. By lifting the exhaust valve by a small lift Hs in the suction stroke, a fraction of the exhaust gas in the exhaust passage flows back into the cylinder. Thus, internal EGR is performed and generation Nox is reduced.
There are disclosed several types of EGR system of internal combustion engine. For example, in Japanese Laid-Open Patent Application No. 7-133726 (patent literature 1) is disclosed an inlet air control system of an internal combustion engine. According to the system, an inlet control valve is provided in the inlet air passage. Negative pressure is formed in the inlet air passage by closing the inlet control valve and inlet valve in this order just before the end of suction stroke. When the inlet valve is opened before the inlet control valve is opened just before the end of exhaust stroke, combustion gas in the cylinder flows into the inlet passage retained in negative pressure aided by the upward movement of the piston. The combustion gas flowed into the inlet passage flows into the cylinder together inlet air in the suction stroke as EGR gas. In this way internal EGR is performed. By controlling negative pressure in the inlet passage by in accordance with engine operating conditions such as engine load and rotation speed through controlling the inlet control valve or other means, the rate of EGR gas can be controlled.
In Japanese Laid-Open Patent Application No. 2005-48743 (patent literature 2) is disclosed a control system of a turbocharged engine equipped with after-treatment devices of exhaust gas such as catalytic converters and diesel particulate filter (DPF). In this system, a means for calculating equivalent flow area of the DPF decreased due to deposition of particulate matter in the DPF and a means for controlling nozzle angle of the variable geometry turbocharger in accordance with reduction in the equivalent flow area of the DPF. When exhaust backpressure increases due to the deposition of particulate matter in the DPF, the nozzle angle is increased, i.e. area between the tips of nozzle vanes is increased so that exhaust backpressure is decreased and increase in EGR rate is suppressed.
In an engine equipped with the exhaust valve sub-lift type internal EGR system, the exhaust valve is lifted again in the inlet stroke by the small lift (Hs) as shown in FIG. 6 so that a fraction of exhaust gas in the exhaust passage flows back into the cylinder to be mixed with the charged air. When the flow resistance in the DPF located downstream of the turbocharger increases due to deposition of particulate matter, exhaust backpressure increases and inlet air flow to the engine decreases, as a result excessive exhaust gas recirculation occurs, which enhance generation of particulate matter mainly composed of carbon particles and deposition of particulate matter in the DPF is enhanced. Further, when the internal EGR rate is excessively large, combustion temperature increases and effect of reducing NOx generation by EGR is decreased by the increased combustion temperature.
Particularly, with an engine provided with a particulate filter, when deposition of particulate matter in the particulate filter increases, exhaust backpressure increases due to increased flow resistance in the particulate filter. The increase in exhaust backpressure induces decrease in air supply which induces increase in internal EGR rate.
In the system disclosed in the patent literature 1, internal EGR amounts are controlled by controlling negative pressure in the inlet air passage between the inlet control valve and inlet valve. However, a means to deal with the problem incurred by the increase in combustion temperature due to increased EGR rate is not disclosed.
In the system disclosed in the patent literature 2, the system is applied to an engine equipped with a variable geometry turbocharger and the nozzle angle of the turbine is increased to increase nozzle area in accordance with decreased equivalent flow area in the DPF due to deposition of particulate matter. However, the system is very complicated and it is not easy to control the nozzle angle with high precision, and a means to deal with the problem incurred by the increase in combustion temperature due to increased EGR rate is not disclosed in the patent literature 2.